I ask because the preferred argument from opponents of the death penalty is doubt: We can never be sure; look at all of the people released from death row; we can't afford to risk ending a single innocent life.

None of those arguments apply to Dzhokhar Tsarnaev. He admitted, through his lawyers, that he and his brother murdered three people and maimed 260 others at the Boston Marathon. (A few days later, they murdered a police officer.)

Tsarnaev knowingly left a bomb next to a family on a family outing. Martin William Richard, 8 years old, died. His sister Jane lost a leg. His mother lost an eye.

A half-hour after the bombing, Tsarnaev went to the Whole Foods to buy some milk, and the next day, he wrote on Twitter, "I'm a stress-free kind of guy."

Ever since Rolling Stone's asinine cover story on the murderer, Tsarnaev has become something of a sex symbol for the morally stunted and chronically stupid. If you're one of them, or just someone prone to conspiracy theories who thinks maybe Tsarnaev's confession was coerced, bear in mind that he was captured on video planting the bombs. A jury convicted him on 30 out of 30 counts against him.

In other words, we know he did it. Does he deserve the death penalty?

Wait, before you answer that, consider Michael Slager. He's the North Charleston, S.C., cop who shot Walter Scott in the back as he was fleeing and then allegedly lied about why he did it.

I don't have to say he allegedly shot Scott because Slager admitted that much. I do have to say that Slager allegedly lied because that's probably going to be decided in a courtroom. Slager claimed he was in fear for his own safety after Scott stole his Taser. But it's obvious he lied because the shooting was captured on video. Slager can even be seen apparently moving the Taser to fit his story.

Legally, it's harder to argue that Slager should get the death penalty if convicted. Not all murders are equal before the law. It's unclear how much premeditation, if any, there was in this case. Presumably Slager didn't know Scott before he pulled him over for a traffic stop.

Still, I think you could make a case for the death penalty in cases like this.

The analogy that comes to mind is the wartime military. There are capital offenses for crimes other than murder because the integrity and effectiveness of the armed forces is a priority. We are not a martial society, but I could make a similar argument about police officers who murder and lie about it. Faith in the fairness of the justice system is simply indispensable to a democracy and social peace. Lack of such faith may be why Scott ran from Officer Slager. If so, his mistrust was tragically well-placed.

There's neither the time nor the space to rehearse the whole death penalty debate again. People claim, usually tautologically, that retribution is illegitimate because revenge is illegitimate. Maybe that's true. But it seems to me that what some people call revenge many others see plainly as justice.

Deterrence is often a distraction. Capital punishment clearly doesn't deter every murderer, but does it deter any would-be murder? It seems obvious it must. Deterrence is a red herring because the function of the death penalty isn't simply to scare a would-be murderer with the corpse of a convicted one; it is also to inform an entire society about what we take seriously.

Tsarnaev is, literally, a traitorous, child-murdering cop killer. He became a citizen on September 11, 2012, and by the spring he was plotting to blow up as many Americans as he could. If we can't take that seriously, we can't take anything seriously.

Slager awaits trial and is obviously a less cut-and-dried candidate for the death penalty. But killer cops do more than simply commit murder; they inflict a grievous wound to the integrity of all cops and to the justice system itself.

Slager deserves his day in court. But Tsarnaev had his -- and now he deserves death. It is honorable to oppose the death penalty on moral grounds. But it is dishonorable to blow smoke about uncertainty in other cases when there is certainty in this one.